Not Applicable
Not Applicable
Not Applicable
Grating spectrometers broadly define the optical art to which the present invention relates, particularly compact spectrometers adapted for examining visually the spectrum of the sun. High dispersion, that is, the ability of a spectrometer to show closely-spaced detail as separate, is only advantageous visually if the image of the spectrum conveyed to the eye is bright enough for the eye to perceive the detail. Sunlight can be so highly dispersed and, subsequently, magnified that it appears faint, the fine detail murky. The present invention extends the eye""s dynamic range by ensuring that high, spectral resolution remains matched to good, apparent contrast beyond the usual limits of the dynamic range.
The sun""s visible spectrum may be roughly differentiated into the spectrum of the photosphere and the spectra, quite different, of sunspots. This difference may, in principle, be seen whenever the image of a sunspot is projected onto the entrance slit of a high-dispersion spectrometer, provided only that the sunspot is wider than the slit. It is best seen, however, when the image of the sunspot is made as large as possible relative to the entrance slit""s length, provided only that good contrast is maintained.
In sum, then, visually to exploit high dispersion, as well as to distinguish sunspot spectra, the spectrometer""s light-input device must, of necessity, be a telescope. Telescopic means for intensifying and imaging light, however, particularly if movably mounted and modestly sized, do not couple easily to laboratory, bench spectrometers, which may weigh several kilograms. The present invention, by comparison, is light in weight and very small. Nonetheless, it discloses detail of haunting subtlety throughout the visual range. Its resolution closely approaches the practical limit of its diffracting grating. Solar absorption lines having peaks spaced apart by ≈0.31 xc3x85 (0.031 nm) are easily split. For a spectrometer that fits comfortably into the palm of a hand, that is rather good.
The perception of faint, solar absorption lines is significantly enhanced by the elimination of glare, that is, by avoiding halation of the retina. If brightness is optimized for those wavelengths at which the eye is most sensitive, then the eye""s light-adaptive ability will automatically compensate for reduced brightness throughout the remainder of the eye""s nearly 260 nm, dynamic range. Shown in FIG. 1 (see Warren J. Smith, Modern Optical Engineering, (copyright) 1990 McGraw Hill), the human eye""s photopic (color) sensitivity peaks somewhere between 550 nm and 560 nm, before falling off precipitously below about 435 nm and above about 695 nm (where sensitivity drops to approximately 1% of peak sensitivity), depending on the individual. These xe2x80x9cfall-offxe2x80x9d wavelengths, however, are significant for a solar-related reason, too: they are where many of the most important features of the sun""s spectrum, xe2x80x9cmust-seexe2x80x9d items on any first, educational tour, are to be found. The broadest of all the solar, absorption lines, the resonance, K, line of singly ionized calcium (CaII), lies at 393.4 nm, followed closely by the H line of CaII at 396.8 nm. The most important solar line of all, certainly historically, is H-alpha, written Hxcex1, the first line in the Balmer series of hydrogen, at 656.3 nm. Redward of Hxcex1 are the stunning, telluric lines of O2. Their band head, Fraunhofer""s B line, lies at 686.7 nm.
The human eye""s greatly diminished sensitivity below 400 nm must be compensated, if the H and K lines are to be observed distinctly. If brightness has already been adjusted so that the continuum is free of glare at 560 nm, then below 410 nm the continuum will simply have become too faint for the human visual system dynamically to compensate contrast on its own. Depending on the time of day (i.e. on solar altitude) and on the humidity (terrestrial water vapor absorption), the H and K lines may either not be seen at all, or they may appear as black phantoms against an only-very-faintly luminous, deep-violet background. Typically, to compensate, a spectrometer""s slit will be widened, that is, a tradeoff is made for increased brightness at the expense of diminished resolution. However, a ten-fold increase in perceived intensity requires a roughly ten-fold increase in slit width, and with this widening many fine lines and features near, between, and in the H and K lines are lost to view. The lines"" extraordinary, natural broadening appears narrower than it actually is, as the lines"" albatross-wide, feather-fine wings get merged evermore coarsely into the expediently-brightened continuum. Then, too, the necessary, adjustable slit, will be expensive, especially if it is to be capable of reliably repeating widths  less than 10xcexc (1 micron=10xe2x88x926 m≈0.00004xe2x80x3) while maintaining slit-jaw parallelism.
The present invention eliminates all of these disadvantages.
The present invention, especially in its exemplary embodiment, combines high-resolution with low-cost, small size, and low weight, and in such a manner that any notable improvement in contrast and/or in resolution will require a disproportionate increase in expense and/or in bulk. The present invention""s single, spherical mirror could, for example, be replaced by independently mounted, but far-more costly, toroidal mirrors. The present invention is thus intended to satisfy an unmet, instrumental need among educators, and to supply an IR/UV-shielded, solar spectrometer to the high-end, amateur, astronomical community.
Small, grating monochromators, off-the-shelf units built, for example, by Acton Research or Optometrics, usually are rather fast, commonly xe2x89xa6f/4, which allows them to accommodate, with appropriate, internal baffling, light input from fiber-optic light guides, which ordinarily have high, numerical apertures (0.2 to 0.5) as well as fiber diameters of around 200 microns. U.S. Pat. No. 5,231,461 to Silvergate et al. (1993) shows a collimating mirror illuminated by sunlight input through a fiber-optic xe2x80x9cslitxe2x80x9d.
Numerical aperture, we recall, for an imaging mirror or conventional lens is just one half the reciprocal of its paraxial focal length. Results discussed below first in terms of f/ratio will frequently be converted for ease of reference into the equivalent, numerical-aperture (n.a.) formulation.
In contrast to the typical, small monochromator, a telescope will usually have an f/ratio substantially higher than f/4, say f/8. Such a telescope, if used as the light-input device for an f/4 monochromator, will not illuminate fully the monochromator""s grating, that is, the telescope""s ray cone will be excessively narrow, and so the telescope will fail fully to exploit the grating""s resolution.
In the solar case, where the desire is understandably great to project onto the spectrometer""s entrance slit as large an image of the sun as possible, in order better to isolate sunspot spectra, the apparent mismatch of high-f/ratio telescope as small-monochromator, light-input device is only aggravated. The larger the desired solar image, the greater must be the telescope""s effective focal length. Given the high cost of large-diameter optics, the greater the effective focal length, the higher will be, as a practical matter, the f/ratio.
A monochromator, we note, can easily be turned into a visual spectrometer by first removing the exit-slit and by then installing magnifying optics with which to view the imaged spectrum, normally hidden behind the exit-slit assembly. The one, obvious exception, of course, is the true Littrow mount.
There is yet another reason, why, for a high-dispersion spectrometer in which the image of the spectrum is magnified for viewing, i.e. in which the input light is very-greatly spread out, a high f/ratio for the telescopic, light-input device is unavoidable. Briefly stated, the maintenance of optimum visual contrast, once it has been empirically determined, depends, essentially linearly, on focal ratio.
Using the present invention""s exemplary embodiment as a test bed, experiment demonstrated that visual contrast was optimized at 550 nm when a solar image 6.8 mm in diameter was focused onto the spectrometer""s entrance slit by an objective having an entrance pupil 37.2 mm in diameter. For a fixed slit width, this optimum contrast will be maintained for any, entrance-pupil diameter by holding constant the ratio of entrance pupil area to solar image area, i.e. by holding constant the amplification of light intensity.
The diameter, D, of the solar image depends on focal length alone, and also linearly, equaling 9.42 mm for each meter of focal length, (1.13 inches for each 10 ft), seasonally averaged, so that, for a focal length F, D=9.42xc3x9710xe2x88x923F. Holding constant the ratio of entrance pupil area to solar image area, i.e. maintaining optimum visual contrast, therefore requires, for a telescope of aperture D used on axis and having a central obstruction of diameter d, that
[6.8/37.2]2=Ø{[(9.42xc3x9710xe2x88x923F)/2]2}/Π[(D/2)2xe2x88x92(d/2)2],xe2x80x83xe2x80x83(1)
or
6.8/37.2=[9.42xc3x9710xe2x88x923F/D][1/(1xe2x88x92d2/D2)1/2].xe2x80x83xe2x80x83(2)
Assuming that dxe2x89xa6D/2, we have
(6.8/37.2)(0.75)1/2xe2x89xa69.42xc3x9710xe2x88x923(F/D)optimumxe2x89xa66.8/37.2,xe2x80x83xe2x80x83(3)
or
f/16.8xe2x89xa6(F/D)optimumxe2x89xa6f/19.4,xe2x80x83xe2x80x83(4)
which, in terms of numerical aperture, is
0.0298xe2x89xa7n.a.optimumxe2x89xa70.0258.
Individuals, of course, will vary somewhat in their estimate of optimum contrast, depending particularly on their age and health, so that the results in (4) may vary perhaps by plus-minus 20%. The larger point is that optimum, visual contrast for a high-dispersion, grating, solar spectrometer will always come at a telescopic, light-input f/ratio that is substantially greater, meaning by a factor of around four, than the f/ratio of the typical, small, commercial monochromator.
The amplification of light intensity that produced the optimal visual contrast, ignoring some relatively minor losses due to filtering, was just the ratio of entrance-pupil area to solar-image area, namely (37.2/6.8)2, or about 30 to 1.
Image intensity, all other system parameters remaining unchanged, is roughly proportional to slit width. The f/19.4 focal ratio that delivered the optimum contrast for the exemplary embodiment of the present invention did so for a 5xcexc slit. For a 4xcexc slit, the optimum ratio drops to f/15.5. For a 3xcexc slit it is f/11.6. For a 2xcexc slit it is f/7.8. All of these f/ratios are substantially greater than f/4, the typical, small, commercial monochromator (turned spectrometer) f/ratio.
The present invention takes the apparent, f/ratio-mismatch of small, visual, grating spectrometer to telescopic, light-input device and turns it to advantage, by recognizing that, for a very intense light source, such as the sun, the fast, spectrometer f/ratio can be exploited. If an ultra-narrow slit is installed, and provided that the telescope""s f/ratio is sufficiently high, Fraunhofer diffraction (preview FIG. 5) will broaden the light re-radiated at the slit (in accordance with Huygen""s principle) into a cylindrical wavefront that can be made closely to match the spectrometer""s f/ratio, with very little loss of light intensity. The present invention""s exemplary embodiment employs a 5xcexc (=0.00019xe2x80x3), laser-cut, fixed slit and an f/19.4 telescopic light-input device. Laser-cut slits down to 4xcexc are commercially available, and 2xcexc slits (1.1 mm long) have been manufactured, though they are more expensive.
The seeming, f/ratio-mismatch is furthermore actually necessary if an ultra-narrow slit is effectively to be used at all in a compact, high-dispersion, visual, solar spectrometer. The focal length of a small spectrometer is typically only a few inches. In the exemplary embodiment, it is just 74 mm. At a distance of 74 mm a 5xcexc slit subtends an angle of just 13.9 arc-seconds:
13.9arcsec=2tanxe2x88x921{[(5xc3x9710xe2x88x926)/2]/(74xc3x9710xe2x88x923)}xc3x973600,xe2x80x83xe2x80x83(5)
where the tanxe2x88x921 function is here understood to return degrees, rather than radians. If such a compact spectrometer were to have an f/ratio matched to the f/ratio of the typical, high-end, commercial, refracting telescope used as a light-input device, say f/8, then the spectrometer""s 74 mm focal-length, collimating mirror would have a diameter of only 9.25 mm (=74 mm/8). The well-known, Rayleigh criterion for the minimum, angular resolution for a circular aperture, xcex8min, is just
xcex8min32 1.220xcex/D radians.xe2x80x83xe2x80x83(6)
In arc-seconds, for xcex=550 nm, this is the familiar 5.45xe2x80x3/D, where D is in inches. The resolution of a matched, f/8, 9.25 mm diameter aperture will thus just be 14.9 arc-seconds, but that is too little to resolve a 5xcexc slit.
The f/3.9 collimating mirror in the exemplary embodiment of the present invention (f/3.9 is equivalent to a numerical aperture of 0.128) has a diameter of 19 mm (=74 mm/3.9) and thus a resolution of 7.3 arc-seconds at xcex=550 nm. The choice of a compact spectrometer that is also fast as the core component of the present invention thus assures that the instrument""s collimating mirror will have more than sufficient resolution to distinctly image an ultra-narrow slit at all visual wavelengths.
This consideration is still more important for the spectrometer""s second, image-forming mirror, because, at long wavelengths, it will be only partly illuminated by diffracted rays sent to it from the tilted grating (preview FIG. 6). The Rayleigh criterion for the angular resolution of a grating can be written as
(xcex94xcex8)min=[xcex/(Na xcex4 cosxcex2)]{[(360)(3600)]/2Π} arc-seconds,xe2x80x83xe2x80x83(7)
where xcex2 is the angle of diffraction for the wavelength xcex, Na is the grating width (total number of rulings, N, times the width, a, of each), and xcex4 is the proportion of the grating actually illuminated (see e.g. Optics, by Eugene Hecht, Addison-Wesley, 1990. P.428). For the exemplary embodiment of the present invention at Hxcex1, xcex=656.3 nm, xcex2=66.5xc2x0 (preview FIG. 6), Na=20 cm and is fully illuminated, with the result that (xcex94xcex8)min=17.0 arc-seconds. The imaging mirror, however, solely due to the spectrometer""s geometry, is only 42% illuminated at Hxcex1. Thus the resolution of the imaging mirror will be less than a full 7.3 arc-seconds, will in fact be only 20.7 arc-seconds [=(7.3/0.42)(656/550)].
At the K line, xcex=393.0 nm, xcex2=41.2xc2x0, Na=20 cm and is ≈81% illuminated (preview FIG. 7), with the result that (xcex94xcex8)min=6.5 arc-seconds. The imaging mirror, due this time to partial illumination of the grating, is 65% illuminated; its resolution is 8.0 arc-seconds [=(7.3/0.65)(393/550)].
In other words, with a 5xcexc slit, the imaging mirror of the exemplary embodiment is just slightly below the lower limit of size necessary to reproduce the angular separation of wavelengths achieved by the spectrometer""s grating. Only in a short-focus spectrometer that is also fast will both mirrors be sufficiently large, or very nearly so, to exploit fully even a relatively small grating.
The present invention employs a 2400 line/mm grating, which is also very near to the practical, upper limit of grating, line density for the first-order, visual spectrum. The grating equation may be written as
mxcex/a=2cos[(xcex1xe2x88x92xcex2)/2]sin[(xcex1+xcex2)/2],xe2x80x83xe2x80x83(8)
where xcex1, the angle of incidence, and xcex2, the angle of diffraction, are positive when measured counter-clockwise from the grating normal, m is the order number, and a, as before, is the width of a grating ruling (see the Diffraction Grating Handbook, Richardson Grating Laboratory, at http://www.gratinglab.com. Note that care must always be taken to understand which angle-measurement convention is being used by a particular reference! The grating equation given by Hecht in Optics differs by a sign from this xe2x80x9csamexe2x80x9d, standard equation given by the Richardson Handbook!). Since both the sine and cosine must be xe2x89xa61, this may be re-written, entirely independently of spectrometer geometry, as
(1/a)xe2x89xa6(2/mxcex).xe2x80x83xe2x80x83(9)
For xcex=700 nm, (1/a) must therefore be xe2x89xa62857 lines/mm in the first-order visual. Among commercially available gratings, a 2400 line/mm grating therefore represents a practical limit on grating line density for first-order, visual observation. The first-order spectrum, furthermore, is the order of choice for visual observation of the solar spectrum because its free spectral range is the greatest of any order, independent of grating.
The present invention employs, preferentially, an Ebert, single-mirror, plane-grating mount, for its significant advantages of compactness and low cost relative to two-mirror mounts, such as the Czerny-Turner. A single-mirror, Littrow mount might be used instead, however it requires an entrance slit lying in nearly the same plane as, and very close to, the imaged spectrum (Richardson Grating Handbook, op. cit.). The Littrow geometry reduces greatly the space available for easily inserting optics with which to view the imaged spectrum, which in turn limits an observer""s ability to project a full-disk image of the sun onto the entrance-slit and slit housing, making more difficult the orientation of the slit with respect to the sun""s disk. The Littrow is furthermore prone to scattered light in the imaged spectrum, particularly if the light""s source is intense.
A plane grating illuminated by a straight entrance slit produces curved spectral lines. This well-known phenomenon does not, however, in itself diminish resolution. As William G. Fastie noted in A Small Plane Grating Monochromator, J. Opt. Soc. Am. 42, 641 (1952), long, curved slits do increase throughput but do not increase practical resolution: xe2x80x9cthe resolving power for any short portion of the slit was the same as for any other short portion . . . xe2x80x9d The most effective, practical contributor to poor, visual resolution is in fact oil inadvertently smudged from eyelashes onto the ocular.
The present invention""s exemplary embodiment employs a straight, 3 mm long, entrance slit, of which only the middle 2.4 mm actually contribute to the visual field (preview FIG. 16).
Fastie, in Image Forming Properties of the Ebert Monochromator, J. Opt. Soc. Am. 42, 648 (1952)), derived an approximate formula for the maximum length L of a straight slit capable of yielding resolution indistinguishable from theoretical resolution at a wavelength xcex for a mirror of focal ratio f in an Ebert mount, and despite the Ebert""s well-known astigmatism:
L=10xcexf3.xe2x80x83xe2x80x83(10)
For the exemplary embodiment""s f/3.9 collimating mirror and for xcex=560 nm, L=0.33 mm. This xe2x80x9cshort portionxe2x80x9d is about the width of a large sunspot projected onto the slit.
Fastie measured a line separation of 0.05 xc3x85 in the second-order, visible spectrum using a 3xe2x80x3 grating having 30,480 lines/inch, in an Ebert mount with a 762 mm focal-length mirror and a straight, 2 mm, entrance slit (see Fastie, Abstract to Small Monochromator, op. cit.). The exemplary embodiment of the present invention uses a 20 mm square, holographic grating with 2400 lines/mm, blazed in the visual. Minimum perceptible line separation being inversely proportional both to the total number of illuminated, grating rulings and to the order number, the corresponding, visual resolution for the present invention should be about:
0.19 xc3x85=0.05 xc3x85xc3x97(91440/48000)xc3x97(2/1).xe2x80x83xe2x80x83(11)
This agrees closely with observation. The strong, solar, iron multiplet at 4957.5 xc3x85, shown in FIG. 4, has peaks 0.31 xc3x85 apart with a Rayleigh-criterion-like overlap. The peaks are easily split by the present invention, despite the grating""s not-quite-full illumination at that wavelength. Such resolution is fine-enough to allow delicate distinctions to be readily made, such as between the typical, Gaussian profile of most, solar spectral lines and the xe2x80x9ctriangular formxe2x80x9d resulting from natural line broadening, as seen in the optically-deep, Magnesium line at 5172.6 xc3x85, shown in FIG. 3.
Magnification of the imaged spectrum is accomplished in the present invention somewhat as it is in a prism spectrometer, except that the telescope objective has been replaced by an optical relay, preferably 1:2, followed by the usual, short-focal length eyepiece (here 8 mm). The spectrum is thus magnified approximately 64 times (64=2xc3x9732, where 32≈10xc3x9725.4 mm/8 mm; the closest approach of a human eye to an object usually being considered to be 10 inches). The relay also creates a second focal plane, which facilitates the installation of a movable indicator. The preferred indicator in the present invention is superficially similar to the slide for uranium glass shown in FIG. 11 of U.S. Pat. No. 1,746,083 to Kurtz (1930), but functions entirely differently. Kurtz""s slide, even with its uranium glass removed, would not yet be adapted for use in the present invention.
The telescope (a Questar Maksutov) used to form the sun""s image on the entrance slit of the present invention has focal-length of 1210 mm at the slit and forms a solar image 11.4 mm across. This is about four times the length of the 3 mm slit used in the present invention. Thus, the slit can be swept across the sun""s image in bands that are not overly wide relative to a large sunspot group. A 5xcexc slit, incidently, corresponds to ≈0.8 arc-seconds on an 11.4 mm solar disk, or to about one half the width of a solar granule, under excellent seeing conditions.
Sunspot groups with magnetic fields  greater than 3500 gauss, i.e. generally having areas  greater than 1500 millionths of the solar disk, appear about eight to ten times during an 11-year solar cycle. The iron lines at 6173.3 xc3x85 and 6302.5 xc3x85 and the vanadium line at 6258.6 xc3x85 are particularly susceptible to Zeeman splitting (see Bray and Loughhead, Sunspots, Dover, 1964). When such a large sunspot group is viewed near the center of the solar disk, so that its umbral, magnetic field lines lie parallel to the line of sight, each of these sensitive lines will be split into two, oppositely-polarized components, separated by 0.32 xc3x85, 0.32 xc3x85 and 0.42 xc3x85, respectively. The present invention is entirely capable of revealing Zeeman splitting of this large, albeit quite uncommon, magnitude.
The exemplary embodiment""s 5xcexc entrance-slit is cut by laser into a thin disk of stainless steel. The disk is then sandwiched between much-thicker mounting plates, and the entire assembly is mounted onto the spectrometer. To insure optimum resolution, the slit must remain parallel to the grating rulings. Strong, differential heating of the slit assembly will exert uneven stresses on the slit, warping it, possibly permanently after repeated use, and is thus to be avoided. The telescope forming the sun""s image on the slit will typically be stopped down, as well, thus differential heating of its elements is also to be avoided. Furthermore it is prudent practice always to enforce the rule among non-astronomers never to point a telescope at the sun without the proper filters. For these several reasons, when using the present invention the telescope""s entrance pupil must always be fully covered by a heat absorbing, infra-red filter. The filter in the Schott Glass KG-1 to KG-5 heat-absorbing series that is most absorptive is the KG-5, and so it is the filter of choice. FIG. 1 shows the sun""s irradiance multiplied by grating efficiency, where Littrow efficiency has been chosen as a benchmark. Below this curve is the same data but now showing the effect of the KG-5 filter out into the long, visible red. Light intensity at 700 nm is reduced by almost ⅔ (note the log scale)
At the violet end of the visual range, where light amplification becomes necessary for viewing the H and K lines, the present invention amplifies without sacrificing resolution. To accomplish this goal, a movable condenser is interposed into the beam converging from the telescope""s objective, decreasing the telescope""s effective focal length.
This condenser may also be used, of course, in the far, visible red, where light intensity has been reduced by the KG-5 filter. However, if the condenser is used at the far, red end, then it will also amplify the small amount of far-ultraviolet radiation overlapping the first-order spectrum from the second order. This overlap is very ignored, because the second-order, UV-spill is quite weak. Here, however, the condenser amplifies the UV-spill by a factor of about ten, depending on condenser focal length.
That there is any UV-spill results from the free spectral range of a diffraction grating interacting, so to speak, with the earth""s atmospheric transmittance. Below 290 nm the atmosphere fully extinguishes solar, UV irradiance. Below 320 nm the fall-off in UV irradiance is very steep (for data see the Web site of New Zealand""s National Institute of Water and Atmospheric Research at http://katipo.niwa.cri.nz/lauder/uvinfo.htm). Above 320 nm plenty of UV radiation reaches the ground. As for the eye itself, xe2x80x9cthe cornea transmits radiation from approximately 310 nm . . . to 2500 nm,xe2x80x9d and the lens xe2x80x9cabsorbs much of the light between 300 and 400 nm . . . the absorption maxima of the lens are 370 and 280 nmxe2x80x9d (Physiology of the Eye, by William M. Hart, Jr., M. D., Ph.D., Mosby Year Book, 1992). Thus it becomes prudent to place a long-pass filter in the optical train. With a condenser amplifying xe2x89xa610xc3x97, both the Schott GG-385 and the Corion LG-370 filters will work well. The GG-385 filter, as shown in FIG. 1, produces a steep fall-off in throughput below 400. FIG. 2 shows the effect of the GG-385 on the second-order, UV overlap. Subsequent to amplification by the condenser, the UV-spill is reduced in intensity by the GG-385 filter by a factor of ≈0.0001 below the normal level encountered in an ordinary spectrometer used to view the sun""s spectrum. The optical adhesives, furthermore, that are used to cement lenses and that usually are present in an eyepiece, will absorb the UV-spill still further.
Placing a lens, or lenses, close to the entrance slit of a spectrometer is not per se unusual. Lenses close to slits are found in U.S. Pat. No. 2,630,736 to Beitz (1953) and in U.S. Pat. No. 3,563,659 to Thompson (1971). However, the lenses in both these patents are the only lenses that are used to form the image of the light source. Beitz employs a standard, twin lens condenser set (FIG. 1 and col. 3, lines 9-11), while Thompson uses a microscope objective (col. 2, lines 43-45). In the present invention, by contrast, a first real image is formed by a telescope, and then a second real image is formed by the condenser, which uses the first real image as its virtual object.
Cylindrical lenses have quite commonly been placed in front of slits on spectrometers attached to telescopes, typically to broaden stellar spectra. Already in use in the mid-19th century, the technique is described by Schellen in Spectrum Analysis, the sumptuous, 1872 English edition of his German book. Schellen writes (pp. 269-270) xe2x80x9cMerz, the celebrated optician of Munich, constructs direct-vision spectroscopes of great dispersive power . . . FIG. 138 shows the interior construction of such a spectroscope . . . L is a cylindrical lens employed for stellar observations , but withdrawn for observations on the sun.xe2x80x9d The idea of amplifying still further sunlight already focused onto a spectrometer""s entrance slit in order to compensate for fall-off in the eye""s dynamic range appears to be unusual.
It is thus a primary object of the present invention to provide a highly-compact, grating spectrometer that nonetheless achieves sub-one-third Angstrom resolution for light input at a high focal ratio, thereby allowing significant resolution of spatial detail in the light source to be combined with high wavelength resolution of the emitted light.
It is a further object of the present invention to provide a grating spectrometer having a means for compensating the eye""s dynamic range without sacrificing wavelength resolution.
It is yet a further object of the present invention to provide a visual, solar, grating spectrometer with a first-order, free spectral range that is extended by suppressing second-order UV-spill.
These and still further objects and advantages of the present invention will become apparent from a consideration of the following, detailed specification, drawings, and appended claims.